Special fluid control tools are used downhole in oil and gas wells during cementing of the well casing as well during well stimulation procedures used to improve well production. These tools include squeeze packers which are used both in completion and increasing production from the well. The cement retainer and bridge plug are examples of squeeze packers commonly used to conduct various downhole completion and service operations.
These production and service tools may be either of the retrievable or permanent type. Retrievable tools are those which may be set and released by manipulating the tool using either the drill string, a wireline or hydraulic control. Because of the additional complexity and expense involved in using retrievable tools, permanent tools are often used in the operation. These tools normally are set in place and are removable only by drilling the tool out of the casing, through conventional rockbit or milling tool methods.
Permanent tools must be substantially built to withstand the pressures and temperatures encountered at the subterranean level at which they are used. Typically, the tools must be made of drillable materials capable of withstanding 30,000 to 40,000 tensile stress and temperatures up to 300.degree. F. In relatively deep wells, that is those deeper than 10,000 feet, even higher pressures and temperatures are encountered.
Squeeze packer production and service tools normally include holding devices, commonly referred to as "slips". These holding devices are used to engage the wall of the casing to restrain tool movement under well bore dynamics Further, the tools incorporate "pack-off" seals for sealing the casing annulus. These seals permit separating areas where differential pressures are applied and for isolating areas within the casing from other areas at varying depth.
Although tools are referred to as "permanent", it may be necessary to remove such tools. This is accomplished by drilling through the tool, and circulating the remains of the tool to the surface for removal. To facilitate drillability of tools, cast iron, rather than steel, is used. Some attempts have been made in the past to use magnesium and other exotic metals which have sufficient strength of material properties. Even using these materials, such tools have required considerable effort to remove through drilling. And because drilling time is rig time, such removal is costly. For example, to remove a permanent cement retainer of the type normally used today, 4 to 6 hours may be required, under ideal conditions.
Further, present design squeeze packers require relatively high internal setting forces and, in many applications, require top and bottom slips with associated cone assemblies for expanding these slips for engagement with the casing wall. As a result of their design, these top and bottom slips are inherently dragged or pulled up the side of the well casing during setting procedures resulting in the "dulling" of the slip teeth. Additionally, present designs often permit the upward movement of the packing element and back up rings during setting, causing a "chafing" of the rubber surfaces. It is not uncommon for a given size squeeze packer or bridge plug to travel four to six inches up the well during setting. Because the tool is not restrained from movement in the casing, the force which can be applied "at the tool" is significantly decreased. Further, the prior art designs which incorporate both top and bottom slips, can result in setting of the tool in a skewed or cocked position in the casing. For example, because the top slip is set first, the tool position upon setting may be skewed relative to the axis of the casing. If this situation occurs, proper setting of the bottom slip may be difficult or impossible. While these designs have been generally acceptable, they have not provided the most efficient arrangement for squeeze packers and related drilling and production tools.
Thus, a need exists for a readily drillable squeeze packer requiring a relatively low internal setting force with an improved holding slip and structure for seating such structure and in expanding the seal pack-off assembly.